The invention comprises a process for making phthalic anhydride and, more particularly, by oxidation of either naphthalene or o-xylene.
Phthalic arthydride is produced by catalytic oxidation of naphthalene or preferably o-xylene. Air or a gas obtained by mixing air with recycled off gas is used as oxidizing gas. The lower limit of inflammability is about 1 mol % o-xylene. Earlier processes were operated below the lower inflammability limit. Later, in order to reduce the flow of air and gas, the process was operated in the inflammable range and in recent processes the gas at the reactor inlet contains up to 1.4 mol % or about 70 grams per standard cubic meters of gas. The disadvantage of these processes is the high gas flow, which leads to high compression power, large diameter of the reactor and high costs of separation of the phthalic anydride from the Reaction gas. In another process, more than 50% of the off gas is recycled, mixed with fresh air, whereby a gas containing only 10-11% oxygen is obtained, which is outside of the inflammable range at any content of o-xylene. Therefore this process can be safely operated at 1.8 mol % o-xylene. However even in this latter process the gas flow and the compression power remain almost equally high.
Tubular reactors are used in most cases. The cooling of the reactor is achieved with a molten salt bath. The molten salt flows in the shell side of the reactor generally countercurrent to the direction of gas which flows inside the tubes containing the catalyst. The hot molten salt from the reactor is cooled in an exchanger in which high pressure steam is produced. The molten salt is then recycled to the reactor. The number of the tubes and the reactor diameter are determined by the gas flow and the allowable pressure drop. For example if at a given gas flow the reactor diameter is reduced by only 20% the gas pressure drop doubles. The length of the tubes is then determined by the required mass of catalyst and the quantity of heat to be removed. If the temperature of the salt bath is low, the reaction rate is also low and the conversion unsatisfactory. At too high salt temperature, the peak and the outlet temperature become excessive and the product yield is low. Therefore at a given catalyst mass and tube length the salt bath temperature must be controlled in a narrow range to obtain both satisfactory yield and conversion. The best temperature of the salt melt depends also on the direction of flow. In the first part of the reactor, more heat is produced than removed and the temperature of the gas rapidly rises to its peak level between 440.degree. and 500.degree. C. The largest portion of the feed is converted in the first part of the reactor. In the second part of the reactor the reaction is being completed, the heat removal is predominant and the gas temperature is dropping. A disadvantage of the existing processes is that due to the high gas flow the reactor diameter becomes very large and the transport of the reactor becomes very difficult. In case of large reactors with diameters between 5 and 6 meters the tube sheets are very expensive.